With the Morefunc add in, you can use
=MCONCAT(A1:D1,"_")
The Morefunc add-in is a free download, available at:
this site.
Or load in the ACONCAT Function ( code below ) and substitute ACONCAT for MONCAT in the above.
<font face=Courier New><SPAN style="color:#00007F">Function</SPAN> aconcat(a <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">Variant</SPAN>, <SPAN style="color:#00007F">Optional</SPAN> sep <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">String</SPAN> = "") <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">String</SPAN>
<SPAN style="color:#007F00">' Harlan Grove, Mar 2002</SPAN>
<SPAN style="color:#00007F">Dim</SPAN> y <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">Variant</SPAN>
<SPAN style="color:#00007F">If</SPAN> <SPAN style="color:#00007F">TypeOf</SPAN> a <SPAN style="color:#00007F">Is</SPAN> Range <SPAN style="color:#00007F">Then</SPAN>
<SPAN style="color:#00007F">For</SPAN> <SPAN style="color:#00007F">Each</SPAN> y <SPAN style="color:#00007F">In</SPAN> a.Cells
aconcat = aconcat & y.Value & sep
<SPAN style="color:#00007F">Next</SPAN> y
<SPAN style="color:#00007F">ElseIf</SPAN> IsArray(a) <SPAN style="color:#00007F">Then</SPAN>
<SPAN style="color:#00007F">For</SPAN> <SPAN style="color:#00007F">Each</SPAN> y <SPAN style="color:#00007F">In</SPAN> a
aconcat = aconcat & y & sep
<SPAN style="color:#00007F">Next</SPAN> y
<SPAN style="color:#00007F">Else</SPAN>
aconcat = aconcat & a & sep
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">If</SPAN>
aconcat = Left(aconcat, Len(aconcat) - Len(sep))
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">Function</SPAN>
</FONT>